And I'm David Greene. Good morning. In the midst of the housing crisis in 2008, the mortgage giants Fannie Mae and Freddie Mac were brought into government hands. And today, over 90 percent of mortgages are guaranteed by the U.S. government. That's a potential burden for taxpayers if mortgages fail. Yesterday, a bipartisan Senate bill was introduced to try to unwind the government takeover, as well as Fannie and Freddie. NPR's Yuki Noguchi reports.

YUKI NOGUCHI, BYLINE: By all measures, the housing market is improving. But, as analyst Karen Shaw Petrou notes, it's done so largely on the back of the U.S. government.

KAREN SHAW PETROU: We have a socialized housing finance system, because they set the terms for mortgages. They set the rates. I don't know where free enterprise is.

NOGUCHI: Four and a half years ago, with the housing sector in freefall, Fannie Mae and Freddie Mac were taken over by the U.S. government. The Federal Reserve has been buying mortgage-backed securities ever since to lower interest rates and jumpstart home-buying. No one expected the government support to go on forever. But now Petrou says the question is: How do we wean the market off of it?

PETROU: There are tremendous sum interests here: realtors, homebuilders, lenders, borrowers, mortgage insurers. This is a system with a tremendous amount sum cost, and that's what makes it really hard.

NOGUCHI: But Senators Robert Corker, a Tennessee Republican, and Democrat Mark Warner of Virginia are trying to tackle it. Yesterday, they proposed a bill to replace Fannie and Freddie with a new, federally run insurance system. Under the new system, private investors would absorb losses. Only if losses exceed private funds would a newly formed Federal Mortgage Insurance Corporation step in. Speaking at a news conference yesterday, Senator Corker said the bill balances competing interests.

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REPRESENTATIVE ROBERT CORKER: It lessens the footprint of the federal government in housing. It winds down Fannie and Freddie, but at the same time, it keeps the housing finance industry in a liquid state, something that we believe is good for our country.

NOGUCHI: One of the ways the bill proposes coaxing more private investment into housing finance is by reducing the size of a home loan eligible for a government guarantee. The loan limit would gradually go from its current 625,500 to 417,000, where it was before the crisis. And private lenders would finance all loans above the limit. Tom Deutsch is executive director of the American Securitization Forum, which represents the financial industry. His group supports the bill.

TOM DEUTSCH: It gets the government out of the guarantee business, except in catastrophic cases. The downside for many consumers is that any change to the mortgage system, any elimination of effective government subsidies, does require that mortgage rates will go up.

NOGUCHI: Deutsch acknowledges the bill faces an uphill battle.

DEUTSCH: There will be significant challenge to the politics of this, because the first rule in Washington is that once you have some kind of subsidy, it's very hard to take it out of somebody's hand. But the flipside of it is that there is a pretty strong view in Washington that something has to get done here.

NOGUCHI: One critic of the Corker-Warner proposal is Edward Pinto, a resident fellow at the American Enterprise Institute and a former executive vice president at Fannie Mae.

EDWARD PINTO: This bill suffers from the same flaw that many of the other proposals that have been put out there have.

NOGUCHI: He says the issue is that this latest proposal still relies heavily on government guarantees to try to advance social goals of making housing more affordable. He argues housing policy should instead make sure only the most reliable or prime borrowers should get loans.

PINTO: That's the only way I know of, in my experience, to get a good, stable result that helps not only protect taxpayers, but helps protect homeowners.

NOGUCHI: This is likely only the beginning of a debate about the future of housing. Tomorrow, the Senate Banking Committee holds its confirmation hearing for Melvin Watt. Watt is President Obama's nominee to head the Federal Housing Finance Agency, which oversees Fannie Mae and Freddie Mac. Yuki Noguchi, NPR News, Washington. Transcript provided by NPR, Copyright NPR.